Process for electrochemical deposition of tantalum and an article having a surface modification

ABSTRACT

A process for electrochemical deposition of tantalum on an article in an inert, non-oxidizing atmosphere, or under vacuum, in a molten electrolyte containing tantalum ions, comprising the steps of: immersing the article into the molten electrolyte heated to a working temperature, passing an electric current through the electrolyte to thereby deposit a tantalum coating on the article, wherein the process of tantalum deposition at least in an initial phase deposits pure α-tantalum.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates byreference essential subject matter disclosed in InternationalApplication No. PCT/DK02/00127 filed on Feb. 26, 2002 and Danish PatentApplication No. PA 2001 00314 filed on Feb. 26, 2001.

FIELD OF THE INVENTION

The present invention relates to a process for electrochemicaldeposition of tantalum on an article in an inert, non-oxidizingatmosphere, or under vacuum, in a molten electrolyte containing tantalumions, comprising the steps of:

immersing the article into the molten electrolyte heated to a workingtemperature,

passing an electric current through the electrolyte to thereby deposit atantalum coating on the article,

using the process of tantalum deposition to at least in an initial phasedeposit pure α-tantalum.

Further the invention relates to an article, such as an implant,provided with a surface modification of tantalum by the above process.

BACKGROUND OF THE INVENTION

The last fifty years a lot of effort has been made to provide a way toprovide articles of e.g. steel with a surface layer of corrosionresistant material, such as tantalum. Tantalum possesses corrosionresistance even at high temperatures, so this material is advantageousin many fields where products are subject to corrosion. Further tantalumis a biocompatible and tissue-friendly material, so it is well suitedfor application in the medical field, such as for implants.

Known techniques for depositing refractory metals, such as tantalum, onan article, involve electroplating in an electrolyte comprising amixture of fused salts, the mixture including a salt or salts of therefractory metal with which the article is to be plated. WO 98/46809discloses a method for electroplating with a refractory metal. Accordingto this method a molten electrolyte is used, said electrolyte consistingof refractory and alkali metal fluorides and a melt of sodium, potassiumand caesium chlorides, and electrical current is passed through theelectrolyte in alternating, repeating cycles. Methods for electroplatingusing mixed chloride-fluoride melts have, however, a tendency to resultin porous coatings and coatings having a low adherence. Further caesiumis very expensive, so therefore this method is not suited for industrialapplication.

EP 0 578 605 by the applicant in the present application discloses afused-salt bath and a process for electrolytic surface coating. The bathcomprises an alkali fluoride melt containing oxide ions and ions of themetal to be deposited.

SUMMARY OF THE INVENTION

An object of the invention is to provide a process for electrochemicaldeposition of tantalum depositing smooth surface modifications that donot crack or peel off.

The process according to the invention is characterized in applying theelectrical current in a first type of cycles in a nucleation phase, saidfirst type of cycles comprising at least one cathodic period, followedby at least one anodic period, and a pause.

Tantalum is an allotropic material having two different kinds of latticestructure: α-tantalum and β-tantalum. It is found that α-tantalum,having a body-centered cubic lattice, is more ductile than β-tantalum,having a tetragonal lattice. β-tantalum is relatively hard (2000 Knoopversus 300–400 for α-tantalum) and there is a risk that a layer ofβ-tantalum may crack and/or peel off, either during the process or inuse later. A visual inspection of the surface reveals whether thetantalum deposited is α-tantalum or β-tantalum, as α-tantalum isblue-grey, while β-tantalum is lighter in colour and greyish. Asmentioned α-tantalum is ductile, and this means that a deposition ofα-tantalum on a surface of a base material can follow any normalmovement of the base material, e.g. because of difference in thermalexpansion of tantalum and the base material. By initially depositing apure structure of α-tantalum onto the surface of the article there isobtained a coherent ductile surface modification onto the base materialresulting in minimum risk of cracks and/or peeling, and therefore thecorrosion resistance and biocompatibility for an article modified withα-tantalum is high, even if the article is used in heavy duty.

An electrolyte based on a mixture of chlorides and fluorides can beused, but using a electrolyte based on a melt of fluorides andcontaining substantially no chlorides and having a ratio of refractorymetal to oxide content higher than 3:1 is found to be advantageous, asthe resulting deposition to a higher degree is smooth and non-porous,and the α-tantalum more easily obtained.

Although direct current can be used, it is found to be advantageous toapply the electrical current in a first type of cycles in a nucleationphase, said first type of cycles comprising at least one cathodicperiod, followed by at least one anodic period, and a pause. The use ofalternating cathodic and anodic periods reduces the risk of dendriteformation and growth, and further reduces the risk of unevendistribution of metal ions and uneven current density associated witharticles of complex geometry. It is found that a pause after the anodicperiod will stimulate the formation of α-tantalum, presumably becausethe pause leaves time for the species to convert and nucleate asα-tantalum.

By applying the current during the nucleation phase with a currentdensity above the current density of deposition of β-tantalum, but belowthe limiting current density, it is found that pure α-tantalum can beachieved.

In electrolysis, the charge on the electrode surface is always balancedby attracting ions of opposite charge from bulk solution to theimmediate vicinity of the surface. This means that in theelectrode-electrolyte interface two layers of opposite electrical chargeexists: the electrical double layer. The double layer is analogue to anelectrical capacitor, and it takes a certain time to charge anddischarge the double layer. By applying current pulses having durationlonger than a charge/discharge period of a double layer, it is foundthat pure α-tantalum can be achieved.

The duration of the pause is chosen to be longer than the duration ofthe cathodic and anodic period of the cycle, preferably 2–8 timeslonger, in particular 4 times longer. The duration of the pause is acompromise between formation of α-tantalum and the overall duration ofthe nucleation phase.

Another object of the present invention is to provide an article havingimproved corrosion resistance.

To achieve this the surface modification is non-porous and at leastcomprises an interface layer of α-tantalum. α-tantalum is especiallyadvantageous as interface layer, as it has high ductility and hence thislayer will counteract formation of cracks and peeling of the surfacemodification, and further it will seal off the surface.

The surface modification may have any suitable thickness, but it isrecommended that the surface modification has a thickness of about 2–14μm, preferably more than 5 μm and less than 12 μm, and in particular8–10 μm. A thickness of the outer zone of 2 μm may be sufficient bysimple geometrical forms of the base body, but with holes and edges, themodification will result in a thinner outer zone and thus a risk of aporous surface. A thickness of the outer zone of more than 14 μm willentail an increase of the process time and more considerable materialexpenses, and this will thus be unfavourable for economic reasons. Ithas turned out that a thickness of the outer zone of 8–10 μm makes areasonable compromise between certainty of a sufficiently thick outerzone over the entire base body and economy.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be further illustrated withreference to embodiments and the drawing, where:

FIG. 1 is a schematic representation of charge transport andconcentration profile through an electrolytic cell,

FIG. 2 is two voltammograms of a process,

FIG. 3 is a stationary polarisation curve,

FIG. 4 is a chronopotentiogram,

FIG. 5 is a chronopotentiogram,

FIG. 6 is a schematic view of the electrode vicinity,

FIG. 7 is graphs of current pulses,

FIG. 8 is graphs of current pulses,

FIG. 9 shows a graph indicating the quantity of tantalum, which haspenetrated a base body of a Co—Cr—Mo alloy,

FIG. 10 shows an enlargement at 600× magnification in an electronmicroscope of a base body with surface modification,

FIG. 11 is a surface modified surface with pin-holes,

FIG. 12 is a surface modified surface without pin-holes,

FIG. 13 shows a graph indicating the quantity of Co in the surface areaof a tantalum modified base body produced from a Co—Cr—Mo alloy.

FIG. 14 shows a thin section perpendicular to a fracture zone of asurface modified base body, and

FIG. 15 is a view showing a crack in a base body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following examples, the melt used is a mixture of LithiumFluorides (LiF), Natrium Fluorides (NaF) and Kalium Fluorides (KF). Assource of metal ions can be used an anode made of tantalum, however inthe example a platinum anode is used and as source of tantalum ionsK₂TaF₇ is used, e.g. in the form of pellets, and tantalum is present indissolved form as TaF₇ ²⁻ ions in the melt. The tantalum dissociate intoions so tantalum is present in oxidation number five, meaning that thetantalum has lost five electrons, and at reduction is reduced totantalum metal in a five-electron process, i.e. the reduction takesplace in one step from tantalum in oxidation number five (Ta(V)) tooxidation number 0 (Ta(0)), i.e. metal. At the cathode the depositiontakes place as a process of the formTa(V)+5e⁻→Tawhile at the anode the reverse process takes placeTa→5e⁻+Ta(V).

In FIG. 1 a can be seen a schematic representation of charge transportand in FIG. 1 b can be seen a schematic representation of theconcentration profile through an electrolytic cell. The electrolyte iscompletely dissociated in positively and negatively charged ions K⁺,Na⁺, Li⁺, F⁻ and TaF₇ ²⁻. By applying an electrical field these ionsmove by migration, i.e. an electrostatic phenomenon. Migration leads toan increase in concentration of TaF₇ ²⁻ in the anode vicinity and acorresponding decrease in concentration near the cathode, i.e. thearticle to be provided with a deposition of tantalum. The electrodevicinities are called diffusion layers or “boundary layers” (shown withdashed lines, and the area between the two vicinities is called bulk. Itis normally assumed that no substantial change of concentration takesplace in the bulk, as the constant concentration “c” of the reactant(TaF₇ ²⁻) is maintained by a combination of migration, diffusion andconvection. In the anode vicinity a concentration increase takes placebecause of dissolution of the anode and a decrease in the cathodevicinity because of metal deposition, and in these diffusion layersthere is a concentration gradient “g”. The concentration gradient “g”gives rise to diffusion, which is a physical phenomenon. In theelectrolysis the following elements form part of an electrochemicalmetal deposition process:

transport of charge and matter through the bulk,

transport of charge and matter through the diffusion layer,

charge transfer process (reduction of metal ions to metal in theelectrode region),

crystallisation of reduced metal atoms leading to the development ofsurface layer.

One or more of these elements can control deposition of metal:

by kinetic control or activation control, the process is controlled bythe velocity of charge transfer, so in this case the problem is to makethe metal deposit on the cathode, while

by diffusion control, the process is controlled by the velocity ofcharge transport, so in this case the problem is to provide a sufficientamount of metal ions near the surface of the cathode.

The kind of control in any given example is controlled by the currentdensity in relation to the limiting current density. The current densityis the ratio of the current to the area of the electrode, and thelimiting current density is the current density, where the surfaceconcentration of reactant (i.e. in this case TaF₇ ²⁻) falls to zero,i.e. applying a current density greater than the limiting currentdensity results in a depletion of the electrode vicinity with regard toreactants after a certain time, as the electron transfer (or deposition)is faster than the inflow of reactant into the diffusion layer. Afterdepletion of the cathode vicinity, the reduction of TaF₇ ²⁻ iscompletely controlled by diffusion, so the rate of transport of TaF₇ ²⁻to near the surface of the cathode is conclusive for the rate ofdeposition of TaF₇ ²⁻.

As mentioned above, tantalum is an allotropic material, so for tantalumthere exists two kinds of lattice structure. Body-centered cubic latticedenoted α-tantalum and tetragonal, metastable lattice denotedβ-tantalum. It is found that α-tantalum, having a body-centered cubiclattice, is more ductile than β-tantalum, having a tetragonal lattice.β-tantalum is hard and there is a risk that a layer of β-tantalum maycrack and/or peel off, either during the process or in use later. Bymost processes, however, there is a tendency that deposition of theunfavourable β-tantalum will take place—this is true for Physical VapourDeposition (PVD) processes and fused salt processes.

For the tantalum deposition process cyclic voltammograms can be seen inFIG. 2. Voltammograms are achieved by linear variation of theoverpotential and plotting the current in response hereto. The variationis fast, so the process is not in a stationary condition. By applyingthe overpotential (i.e. a deviation of the potential from theequilibrium potential) in a cycle of cathodic and anodic direction,information of the process or the processes can be achieved. In FIG. 2two voltammograms are shown, one being obtained in the potentialinterval [−600 mV;+1000 mV] and the other in the interval [−800 mV;+1000mV]. In the first one a cathodic shoulder can be seen at −475 mV (shownwith an “a”) and in the second one, a cathodic peak can be seen at −620mV. By X-Ray Diffraction, it can be seen that the metal deposited at theshoulder at −475 mV is β-tantalum, and the metal deposited at the peakat −620 mV is α-tantalum.

Further information can be achieved by stationary polarisation curvesachieved by an increase of the overpotential so slow that stationarycondition is maintained. Stationary polarisation curves provideinformation on the number of processes taking place and the limitingcurrent densities for the processes. The stationary polarisation curvesin FIG. 3 are obtained at an increase of the overpotential of 2 mV/s, sothe stationary condition is maintained, and it can be seen that twoprocesses take place. One process is seen to begin at −90 mV and anotherone at −110 mV, and by X-Ray Diffraction it can be found that theseprocesses are deposition of β- and α-tantalum, respectively. Furtherincrease of the current density results in an approximately linearcurve, still with deposition of α-tantalum. At approximately −800mA/cm², however, the slope of the curve is minimum, indicating that thelimiting current density has been reached. That the slope of the curveis not horizontal at the limiting current density is due to the factthat the area of the electrode increases because of the metaldeposition.

FIGS. 4 and 5 are chronopotentiograms, where a current step is appliedto the electrolytic cell. In FIG. 4 a current density of −1200 mA/cm² isapplied, i.e. a current density greater than the limiting currentdensity, and it can be seen there are two plateaus, one at −140 mV andone at −180 mV, indicating the two processes take place. It can be foundthat which are the overpotentials for deposition of β- and α-tantalum,respectively. It can be seen that at the applied current densitydiffusion control for deposition of β-tantalum takes place after 360 ms,while diffusion control for deposition of α-tantalum takes place after 1s, as at these points there is an increase in potential showing that thelimiting current density of the process referred to is reached. Thechronopotentiogram according to FIG. 5 is obtained at a current densityof −300 mA/cm², i.e. a current density much lower than the limitingcurrent density of −800 mA/cm² as mentioned above. In this figure aplateau at −90 mV can be seen. After 800 ms this process is diffusioncontrolled, and after 2.7 s a potential of −110 mV is reached.

By comparison of FIG. 3 and FIG. 5, it can be seen that the process ofdeposition of β-tantalum takes places at diffusion control, while theα-tantalum deposition process is not diffusion controlled.

FIG. 6 is a schematic view of the electrode vicinity. On the side of theelectrolyte, the partial charge is shielded in that in the near boundary(vicinity) of the electrode an excess of ions of opposite charge isaccumulated. In the electrode-electrolyte interface two layers ofopposite electrical charge exists: the electrical double layer. Thedouble layer is analogue to an electrical capacitor, and it takes acertain time to charge/discharge the double layer. As can be seen inFIG. 7, the charge/discharge of the double layer cause the curves of theFaradaic current to smoothen. FIG. 7 a is a representation of thecurrent applied as a step, whereas FIG. 7 b is a plot of the currentimpressed and the Faradaic current, i.e. the current available to thereduction process. As can be seen, the Faradaic current is not identicalto the current impressed, as the charge and discharge of the doublelayer takes time. As mentioned the Faradaic current is the currentavailable for the reduction process, so this means that pulse effect isreduced. Regarding the lower limit of the pulse duration, it appliesthat the pulse duration must be longer than the charge/dischargeduration of the double layer. In FIG. 7 c is shown a situation, wherethe charging time is so long that the current impressed is not evenachieved within the step period, and in FIG. 5 d a situation is shown,where current steps are applied at a rate faster than the charge anddischarge period of the double layer, so the steps are almost completelysmoothened out.

The prerequisite for a cathodic metal deposition process to take placeis that the metal ions are present in sufficient proximity of thecathode for electron transfer to take place. The prerequisite fordeposition of metal ions on the cathode to take place evenly distributedis that the metal ions prior to the reduction process is correspondinglyevenly distributed around the cathode. Further the even distributionmust always be maintained during the electrolysis.

Formation of dendrites is caused by the phenomena that surface roughness(irregularities) is magnified during the deposition process because ofincreased current density right there compared to the rest of thecathode surface. Coatings without dendrites can be achieved bydeposition under kinetic control.

A way to ensure sufficient amounts of metal ions near the electrodes andeven distribution of metal ions is by agitation of the electrolyte orrotation of the cathode. Another way is to periodically changing thedirection of the current, so that the article to be coated alternatelyis cathode and anode. By periodically changing direction of theelectrolysis current, the electrode concerned will periodically operateas anode from where metal deposited again is dissociated into ions.Hereby the surface concentration, dependent on the pulse period and thecurrent density, can be increased so much that it can exceed the bulkconcentration, which increases the chance of avoiding depletion of theelectrode vicinity. As can be understood, it is possible by pulsing theelectrolyte current to influence the conditions of diffusion andelectron transfer velocity towards kinetic control, and therebyinfluencing the deposition process positively regarding avoidingdendrite growth.

Unlike direct current electrolysis, where the electrolysis currentdensity must be considerably smaller than the limiting current density,pulsed electrolysis provides greater choice of electrolysis currentdensity, and thereby greater possibility to optimise the depositionprocess, while meeting the requirement of kinetic control. By pulsingthe electrolysis current, by periodical current interruption, a doublediffusion layer is established at the interface between bulk andelectrode surface. The reason for this is that only the concentrationconditions in the nearest vicinity of the electrode is changed by fastpulsing (i.e. by pulsing faster than the time needed for establishingthe stationary diffusion layer), while the concentration profile fromthis vicinity outwards is identical to the stationary profile by thecorresponding average current density. During the pulse pauses ions areconstantly supplied to the nearest cathode vicinity, without bydeposition a removing takes place. Thereby a concentration profilevarying in time is created, this concentration profile varying with thepulse frequency; a pulsating diffusion layer being thinner than thestationary diffusion layer. The increased surface vicinity concentrationby the end of a pulse pause gives the opportunity of a correspondingincrease of pulse current density, without the surface concentrationbecome zero.

An important detection is that β-tantalum will not deposit onα-tantalum, i.e. in a nucleation phase it is important to ensure theright parameters to avoid deposition of β-tantalum, but after a coveringsurface modification of α-tantalum is formed, there is no risk ofdeposition of β-tantalum, so focus can be on parameters to avoidformation and growth of dendrites.

In an example, four cylinders made of vitallium were treated by theprocess, the area of the cylinders being 178 cm². The concentration ofthe electrolyte was 3.5 mole % tantalum (oxidation number 5), and theworking temperature was 700° C. For application of current pulses apotentiostat/galvanostat was used (Solartron 1286 ElectrochemicalInterface; Schlumberger Technologies). A program for treatment of thecylinders involve an initial 20 minute warm up phase, where thecylinders are placed in the electrolyte, but no current is applied, anucleation phase taking 20 minutes and applying current in a firstseries of special cycles, and a phase of building up of the surfacemodification taking 38.30 minutes and applying current in a secondseries of special cycles.

The nucleation phase involves 30 cycles of 40 s each. As can be seen inFIG. 8( a) each cycle comprises a first cathodic period of duration of 2s, a 10 s pause, a second 3 s cathodic period, a 1 s anodic period, anda pause of 24 s. In the cathodic period the current applied is 8 A, andin the anodic period 11 A.

The second series of special cycles for building up of the surfacemodification involves 3850 cycles of 600 ms each. As can be seen in FIG.8( b), each cycle comprise a cathodic period of 500 ms and a anodicperiod of 100 ms, the current applied being 8 A and 11 A, respectively.

The current applied in the anodic period is chosen to be higher than thecurrent applied in the cathodic period. This is because a higher currentin the anodic period of the cycle will increase the polishing effect ofthe anodic period of the cycle, where metal is removed from the surfaceof the article, as the higher current will concentrate at irregularitiesof the surface, such as dendrites.

The duration of the application of current in the cathodic and anodicperiod of the cycles is chosen so that the ratio of the amount ofelectrical charge in the cathodic period to the amount of electricalcharge in the anodic period is 4:1. This ratio seems to provide anappropriate compromise between speed of the process and quality of thesurface modification. Higher ratios will of course result in rapiddevelopment of the surface modification, but with an increasing risk ofdendrite formation and pinholes.

The process alloys tantalum into the surface and this means that a metalbody in a hardened form is exposed to a process which alloys tantaluminto the surface of the base body, and that the surface thus has analloy zone, the tantalum diffusing up to some micrometers into the body.The application of the tantalum continues until it forms an outer zonewith a uniform, diffusion-tight surface of essentially pure tantalum.The outer zone proceeds gradually to the alloy zone, which isstructurally anchored completely in the base body. The outer zone has ahigher ductility than the metallic base body, i.e. that the outer zonehas higher deformation ability than the base body such that the outerzone can be extended longer than the base body.

When producing metal articles, micro and macro cracks are formed on thesurface of the metallic base body, and these cracks cannot be completelyremoved by subsequent treatment, even by polishing with e.g. diamondpaste. Furthermore, the surface of the base body will be provided withgrain boundaries, and both grain boundaries and cracks cause notcheffect during fatigue, thus facilitating the initiation of crack growthfrom the surface of the base body, which may lead to fractures, andfurther the cracks may be subject to corrosion. As the outer zone isuniform and impervious, all cracks and grain boundaries on the surfaceof the base body is efficiently sealed. The implant surface is free fromnotch effect, the surface being without grain boundaries, cracks oranything from where a crack may initiate, which entails that the fatiguestrength and corrosion resistance is substantially increased.

These advantages apply to all metal articles. Regarding implants thereis a further advantage that a diffusion-tight outer zone of abiocompatible material is provided. Further the fatigue strength is ofcrucial importance to the durability of implants as most implants areexposed to repeated load. A hip implant at ordinary walk will thus beaffected about once per second, which means that for a person being outof bed for about 5 hours a day, the total number of loads in a year willbe more than 6.5 million. Further, the higher ductility of the outerzone in relation to the base body means that this zone follows themovements of the base body and does not peel off.

An impervious surface without micro porosities is advantageous asbacteria have more difficulties in adhering to an impervious surface,and there is thus less risk of introducing bacteria when inserting theimplant. This means that the healing process is not impeded by bacteriaand the risk of complications is minimized.

According to a preferred embodiment, the metallic base body is aCo—Cr—Mo alloy which has proved to obtain a particularly high increaseof fatigue strength by surface modification.

In a preferred embodiment, the base body is modified by a fused saltprocess to a thickness of the outer zone of about 2–14 μm, preferablymore than 5 μm and less than 12 μm, and in particular 8–10 μm. Athickness of the outer zone of 2 μm may be sufficient by simplegeometrical forms of the base body, but with holes and edges, themodification will result in a thinner outer zone and thus a risk of aporous surface. A thickness of the outer zone of more than 14 μm willentail an increase of the process time and more considerable materialexpenses, and this will thus be unfavourable for economic reasons. Ithas turned out that a thickness of the outer zone of 8–10 μm makes areasonable compromise between certainty of a sufficiently thick outerzone over the entire base body and economy.

A base body 1, shown in FIG. 10, is lowered into a bath of melted saltto be covered by a material. Not all metals can withstand being loweredin a salt melt as the melt is strongly reactive, and thus e.g. titaniumwill be dissolved in a moment (fluoride salt melt). As can be seen inFIG. 10, a uniform diffusion-tight outer zone 2 of tantalum can beobtained with a thickness of the outer zone 2 of about 8–10 μm, and aso-called smooth surface is obtained which is completely even and smoothwithout grain boundaries by this process at appropriate control ofelectric impulses. An even and smooth surface is advantageous as thereis a minimum risk of bacteria on the surface when inserting the implant.

By the fused salt process, an alloy of the modification material isobtained in the surface of the base body 1, the modification materialdiffusing a little into the base body 1. This is seen from, amongothers, FIG. 9 showing a measurement of the content of tantalum atdifferent distances from the surface of a base body produced fromCo—Cr—Mo and surface modified by tantalum. In FIG. 9, a distance of zerorepresents the surface of the base body 1, negative values positions inthe base body 1, whereas positive values represent positions in theouter zone 2. It can thus be seen that in a depth of 100 nm (0.1 μm),there is a weight percentage about 40 of tantalum. This indicates thateven in a base body made from Co—Cr—Mo having a rather closed surface,an alloying of tantalum takes place in the base body 1 which assures acomplete anchoring of the outer zone 2, and thus that the outer zone 2does not peel off.

FIG. 10 shows a sectional view of a surface modified base body 1produced from a Co—Cr—Mo alloy modified by tantalum by a fused saltprocess. The outer zone 2 on the base body 1 has in this embodiment athickness of about 15 μm. It has turned out that the thickness of theouter zone 2 when modified by tantalum by the fused salt process doesnot need to be larger than about 10 μm, however, there is nothing toprevent much thicker outer zones 2, e.g. of 50 μm.

As mentioned, it is essential that the outer zone 2 is uniform anddiffusion-tight which may be difficult to obtain, especially as there isa risk that pin-holes will appear in the surface, this means that theouter zone 2 is provided with through-going holes. This is seen in FIG.11 showing the surface of a surface modified base body. The black spotsare such pin-holes. FIG. 12 shows a surface of a corresponding surfacemodified base body, and it can be seen that this surface is imperviousand without pin-holes.

Since the implant according to the invention has a uniform anddiffusion-tight outer zone, a diffusion barrier is thus provided toassure that unwanted substances in the base body, such as cobalt, do notdiffuse out of the implant. As can be seen from FIG. 13 indicating themeasured quantity of cobalt in the base body, alloy zone and outer zoneof a tantalum modified base body made from a Co—Cr—Mo alloy, themeasured quantity of cobalt reduces in the alloy zone from approx. 65%in the base body 1. Again a distance of zero represents the surface ofthe base body 1, negative values positions in the base body 1 and thealloy zone, whereas positive values represent positions in the outerzone 2. In this connection it should be remarked that the figure due tomeasuring technical limitations provides a somewhat misleading picture.In fact, cobalt is only present in the alloy zone where the quantitygradually approaches zero, whereas no cobalt is found in the outer zone.

FIG. 14 shows a thin section perpendicular to a fracture on acorresponding test piece. It is seen that the outer zone 2 did notloosen or peel off, however, it seems that the outer zone 2 of puretantalum has yielded just at the fracture, which confirms that the outerzone 2 does not peel off and that the outer zone 2 has a higherductility than the base body 1.

FIG. 15 showing an enlargement of a base body 1 of stainless steel whichhas been surface modified by tantalum, is an example of a crack whichseems to stop in the outer zone 2 of tantalum, which may be due to thefact that the outer zone 2 has a higher ductility than the base body 1,the concentration of stress at a crack tip being reduced, and that thereare compressive stresses in the surface of the implant.

1. A process for electrochemical deposition of tantalum on an article inan inert, non-oxidizing atmosphere, or under vacuum, in a moltenelectrolyte containing tantalum ions, comprising the steps of: immersingthe article into the molten electrolyte heated to a working temperature,passing an electric current through the electrolyte to thereby deposit atantalum coating on the article, using the process of tantalumdeposition to at least in an initial phase deposit pure α-tantalum,wherein applying the electrical current in a first type of cycles in anucleation phase, said first type of cycles comprising at least onecathodic period, followed by at least one anodic period, and a pause. 2.A process according to claim 1, whereby using a electrolyte based on amelt of fluorides and having a ratio of refractory metal to oxidecontent higher than 3:1.
 3. A process according to claim 1, whereinduring the nucleation phase the current is applied with pulses having acurrent density of between the limiting current density and the currentdensity of β-tantalum deposition.
 4. A process according to claim 1,wherein the current pulses have a duration longer than acharge/discharge of a double layer.
 5. A process according to claim 1,wherein the duration of the pause is longer than the duration of thecathodic and anodic period of the cycle, preferably 2–8 times longer, inparticular 4 times longer.
 6. A process according to claim 1, whereinthe current applied in the anodic period is higher than the currentapplied in the cathodic period.
 7. A process according to claim 1,wherein the amount of electrical charge in the cathodic period is 1.2 to8 times larger than the amount of charge in the anodic period,preferably 4 times larger.
 8. An article, such as an implant, providedwith a surface modification of tantalum by a process according to claim1, wherein the surface modification is non-porous and at least comprisesan interface layer of α-tantalum.
 9. An article according to claim 8,wherein the surface modification has a thickness of at least 2 μm. 10.An article according to claim 8, wherein the surface modification has athickness of about 2–14 μm, preferably more than 5 μm and less than 12μm, and in particular 8–10 μm.